The present invention relates to an improved balun transformer used, for instance, to match the impedance of an antenna input circuit, and using a common mode coil for noise removal by coupling the common mode coil either at the input side or at the output side of the transformer.
With the conventional balun transformers 10 used for impedance conversion, as shown in FIG. 7 and FIG. 8, coils L.sub.1, L.sub.2, L.sub.3 and L.sub.4 are formed by winding parallel two-strand cables 14 around a spectacles-type core 12. Conversion of impedance (for example, between 300.OMEGA. and 75.OMEGA.) and balanced-unbalanced conversion (for example from a balanced state to an unbalanced state) is performed by wiring the subject coils between the 300.OMEGA. ends 16a and 16b as well as between 75.OMEGA. ends 18a and 18b.
The common mode coil 20 for removing noise (for example transmission noise), as shown in FIG. 7 and FIG. 8, is configured in such a way that the coils L.sub.5, L.sub.6, L.sub.7 and L.sub.8 are formed by winding parallel two-strand cables 24 around the spectacles-type core 22, separate from the spectacles core 12 for the balun transformer 10. The common mode coil is wired so that a signal (for example, one of 40 MHz to 960 MHz) incoming to the input terminals 26a and 26b from a 300.OMEGA. feeder line (not shown) is output at the output terminals 28a and 28b with the common mode noise component of the signal removed.
This conventional system is so designed that the output terminals 28a and 28b of the common mode coil 20 are connected to the 300.OMEGA. ends 16a and 16b on the input side of the balun transformer 10 to thus remove the noise and match the impedances.
However, in the conventional balun transformer 10 arrangement a parallel two-strand cable 24 is wound around the spectacles-type core 22, separate from the core 12 to form the common mode coil 20 for noise removal. Thus, when the balun transformer is coupled with the common mode coil 20 the resulting structure is too large to be incorporated into compact electric devices. Further, such a configuration leads to an increased number of components, increased manufacturing time and higher production costs. Also, the inherently longer connections between the balun transformer 10 and the common mode coil 20 are problematical in that noise is more likely to remain with the signal.